Fig. 6.

Urgency induces graded changes in peripheral muscle activation. a–c Electromyographic (EMG) activity in the response-executing thumb. a Distributions of “motor times,” quantifying the time lag between muscle activation onset and action completion (button click), revealing shorter lags under Speed (red) than Accuracy (blue) emphasis. b Muscle activation (mean EMG spectral amplitude over 10–250 Hz measured in 100 ms-time windows) time-locked to the button click response is increased under speed pressure, and for fast (thick traces) compared to slower (thin traces) responses. c Muscle activation during response execution (100 ms-time window before click, shaded gray in b is increased under speed pressure across all response time bins (Linear mixed-effects model: Regime: χ²(1) = 18.1, p = 2.1 × 10⁻⁵, Supplementary Table 2c), and decreases significantly over reaction time bins (RT: χ²(1) = 5.4, p = 0.020; RT²: χ²(1) = 11.3, p = 0.0008). Error bars indicate S.E.M. across 16 subjects. d–f EMG activity in the response-withholding thumb. d In the response-withholding thumb, the probability of a muscle activation onset occurring without triggering a motor response (partial burst) is increased under speed pressure across a broad time range. e Mean spectral amplitudes measured in two response time bins show that, especially for late responses (thin traces), response-locked traces of muscle activation in the response-withholding hand show increased activation under speed pressure (red traces). f With increasing response time, muscle activity in the response-withholding thumb increases both under Speed and Accuracy emphasis in a time window just prior to the mean EMG onset time in the response-executing thumb (−225 ms to −125 ms, shaded in e). Note that activation in the response-withholding hand is plotted on a much smaller scale than that of the response-executing hand. Error bars indicate S.E.M. across 16 subjects